The present invention relates to an optical disk having one or two recording layers and a method of producing such an optical disk, exhibiting enhanced focusing-error free characteristics at high rotating speed.
High-density optical disks have been developed with the help of high aperture ratio for lenses and short wavelength for lasers.
The next generation optical disks require a thin optical transparent layer coated thereon, to achieve a short distance between an objective lens of an optical pickup and the disk surface, for suppressing lens astigmatism. This is because a large astigmatism will be expected when an optical disk is inclined on a recording/reproduction apparatus due to warpage etc., in proportion to the distance between the objective lens and the disk surface.
Also required to the next generation optical disks is a tolerable range of focusing errors, which occur due to disk-surface vibration.
A method of producing optical disks which overcome such drawbacks is disclosed in Japanese Unexamined Patent Publication No. 2000-311392.
In summary, an optically transparent polymer sheet is annealed at temperature in the range from −20° C. to +80° C. against the polymer thermal-deformation temperature. A recording and a bonding layer are formed on a substrate in this order via the annealed polymer sheet attached to the bonding layer. The bonding layer and the attached polymer sheet constitute an optical transparent layer. Bonding the polymer sheet to the bonding layer decreases winkles formed on the sheet, thus offering a uniform thickness to the optical transparent layer.
Discussed with reference to FIG. 1 is focusing errors due to disk-surface vibration at 10 KHz or lower in disk rotating speed, due to the fact that errors could be indirectly measured through reproduced signals from an optical disk at several ten 10 KHz or higher.
The frequency characteristics shown in FIG. 1 indicate disk-surface vibration against disk rotating speed at 5.3 m/s in linear velocity in reproduction (the same characteristics observed in recording).
The ordinate and the abscissa in FIG. 1 are the disk-surface vibration and the disk rotating speed, respectively, both on logarithmic scale.
A critical line R in FIG. 1 indicates the critical border at which focusing errors will occur. The zone P below the critical line R is a focusing-adjustable zone in which disk-surface vibration can be suppressed by a servo circuitry of an optical pickup, thus accurate focusing on the signal-recorded region on an optical disk being achieved. On the contrary, the zone Q above the critical line R is a focusing-unadjustable zone in which disk-surface vibration cannot be suppressed by the servo circuitry, thus accurate focusing on the signal-recorded region being not achieved.
Observed in FIG. 1 is that the disk-surface vibration exhibits frequency dependency on disk rotation frequency in each of the following ranges: range (A) from 1 Hz to 36 Hz, range (B) from 36 Hz to 1100 Hz and range (c) from 1100 Hz to 10000 Hz.
The frequency characteristics teach the followings:
In the range (A) from 1 Hz to 36 Hz, focusing is adjustable against disk-surface vibration up to 100 μm.
In the range (B) from 36 Hz to 1100 Hz, focusing-adjustable disk-surface vibration is linearly decreased as the disk rotation frequency is increased. In detail, focusing is adjustable against disk-surface vibration up to 100 μm at frequency in the range from 1 Hz to 36 Hz whereas adjustable up to 0.09 μm at 1100 Hz.
In the range (C) from 1100 to 10000 Hz, focusing is adjustable against disk-surface vibration at 0.09 μm or lower.
As understood from FIG. 1, focusing errors can be corrected little at high disk rotation frequency, particularly, it cannot be corrected if disk-surface vibration is not suppressed to 0.09 μm or lower in the range (C).
Optical disks produced by the method disclosed in Japanese Unexamined Patent Publication No. 2000-311392 exhibit less variation in film thickness for optical transparent layers whereas exhibiting disk-surface vibration over 0.09 μm, thus focusing unadjustable.